It is known that high pressure polyethylene, which can be used to prepare high quality cable insulating compositions, can be crosslinked with vinyltrimethoxysilane. This process, however, has the particular disadvantage that vinyltrimethoxysilane is highly volatile, so that significant losses occur during processing, which has the effect of polluting the environment and, because of the toxicity of vinyltrimethoxysilane, also involves a safety hazard. On the other hand, undesirable fluctuations in the quality of the crosslinked product with respect to its dielectric properties and its mechanical values, depending on the crosslinking density and the homogeneity, are caused by the volatility of vinyltrimethoxysilane during processing. Therefore, attempts have already been made to use vinyl-tris-2-methoxyethoxysilane in place of vinyltrimethoxy-silane, and better results have been achieved in some instances. However, a very significant disadvantage is the extreme toxicity of the 2-methoxyethanol formed during the crosslinking reaction. Therefore, the tendency is to abandon this crosslinking process because of the excessively high, long-term toxicological risks. Moreover, the electrical industry now makes more stringent demands with respect to the dielectric, thermal and mechanical properties of the crosslinked cable insulating compositions than it has heretofore been possible to achieve.
For a number of applications where the demands with respect to thermal properties are more stringent, for example for hot water pipes, crosslinking of low-pressure polyethylene is important for improving the dimensional stability. However, no practicable solution has up to now been found for this problem.